(a) Field of the Invention
The present invention relates to an optical power amplifier and a control method thereof. More specifically, the present invention relates to a method for controlling an optical gain and an output power level in a fiber amplifier with a 3-stage optical gain block.
(b) Description of the Related Art
As a result of fast developments in Internet technology, the demand for enhanced transmission capacity has greatly increased. WDM (wavelength division multiplexing) optical transmission systems have been proposed as a way to satisfy such a demand for greater transmission capacity.
An essential requirement of the optical transmission system is the fiber amplifier, which amplifies optical signals so that they may be transmitted a greater distance. The EDFA (Erbium doped fiber amplifier) and the FRA (fiber Raman amplifier) have been extensively used in recent times for amplifiers of the WDM transmission system because they have wide gain bandwidths. In addition to transmission capacity, present-day optical transmission systems require flexibility as network systems, that is, they require control of transmission capacity. Transmission capacity is determined by the number of signal channels and, ideally, the systems are not affected by the add and/or drop of channels that are being transmitted.
In practice, however, the gains of conventional fiber amplifiers of the WDM optical transmission system are varied when the number of signal channels is varied to change input optical power of the amplifier. To solve this problem, an AGC (automatic gain control) function for the fiber amplifier is required to uniformly maintain gain levels.
The transmission span loss may be altered because the transmission span is differently configured or because of other environmental factors. When the transmission span loss is changed, so is the input of the amplifier and, ultimately, the corresponding output of the amplifier. Accordingly, there is the additional need for an ALC (automatic level control) function that enables a constant output to be obtained even when the input is varied because of span loss change.
Many transactions have been published and patents realized that disclose results of obtaining the AGC and the ALC functions in fiber amplifiers (in particular, in the EDFA). K. Motoshima et al. realized the AGC function by monitoring inputs and outputs of each gain block of the EDFA with a 3-stage gain block, and controlling the optical power of pump LDs (laser diode) by an AGC circuit for each gain block (refer to IEEE Journal of Lightwave Technology, Vol. 19, No. 11, pp. 1759˜1767, November 2001). A method for realizing the ALC function is disclosed in this transaction in which power of a specific channel is filtered at the final output to measure the optical power, and a variable optical attenuator provided between the first and second stages is controlled so as to provide a constant optical output power. Although many types of AGC and ALC methods have been disclosed in addition to the above-described method, the method for controlling the output power of pump LDs, for example, as disclosed by K. Motoshima is nevertheless considered to be the most stable and reliable method.
However, since it is necessary, in order to realize AGC and ALC with the use of this method, to provide an additional circuit for each gain block and control gain and output optical power levels so that they remain constant, a corresponding amount of complexity in the circuit results to thereby increase costs. Further, the control circuit needs to be very fast (<5 usec) to suppress the transient effects sufficiently in surviving channels at the output of the amplifier.